Processes for the catalytic dehydrogenation of hydrocarbons are well known. Paraffins having from 5 to 20 carbon atoms per molecule undergo such treatment to form the corresponding olefin. Olefins having from 9 to 16 carbon atoms per molecule are used to alkylate benzene to produce alkylbenzenes, which is an intermediate in the manufacture of detergents. Shorter chain olefins having 5 carbon atoms per molecule are used to alkylate isoparaffins or to etherify alcohols to make motor fuel blending components. A great many other uses for such olefins are known.
Many catalytic dehydrogenation processes use a reactor containing a bed of catalyst. The activity of the catalyst decreases in a gradual manner and the reactor inlet temperature is gradually increased to compensate for this. As this temperature rises, the rate of undesired side reactions will increase, and the selectivity of the process to the desired olefin will suffer. When the temperature rises to an upper limit (e.g., the design temperature of process equipment) or the selectivity of the process drops below a useful level (e.g., the minimum profitable selectivity), the effective life of the catalyst is reached, the “run” of the catalyst is completed, and the catalyst must be replaced.
The effective life of the catalyst typically is approximately inversely proportional to the conversion per pass at which the dehydrogenation process is operated. Operators of these processes select a value of conversion per pass of the feed hydrocarbon through the reactor to optimize operation of the process. This desired conversion per pass will vary with the objectives of the operator. The value may be preselected prior to the start of a run of dehydrogenation catalyst and then maintained throughout the run as the catalyst ages. Alternatively, the value may be varied during the run as the catalyst ages. A process can be operated at a number of different conversions per pass as the catalyst ages and the temperature required to maintain the initial preselected conversion per pass increases. The most important factors in determining the desired conversion per pass are the temperature required to maintain a given conversion per pass as the catalyst ages and the selectivity of the process at a given conversion per pass. The temperature required in a dehydrogenation process to maintain a given conversion per pass depends on the conversion per pass, the composition and condition of the catalyst, the reactants, and other dehydrogenation conditions.
Dehydrogenation processes and catalysts have previously been found to be affected by the presence of water, as described in U.S. Pat. Nos. 3,448,165 (Bloch); 3,907,921 (Winter, III); 5,321,192 (Cottrell et al.); 6,177,381 (Jensen et al.); 6,486,370 B1 (Rende et al.); and 6,756,515 B2 (Rende et al.). U.S. Pat. No. 3,907,921, for example, describes injecting water at an initially optimum value and then increasing the water concentration in the reactant stream as the catalyst becomes less active. U.S. Pat. No. 6,756,515 B2 describes a dehydrogenation process wherein water at less than 1000 wt-ppm based on the hydrocarbon weight is passed to a layered composition in order to decrease the catalyst deactivation rate.
Processes for dehydrogenating hydrocarbons with improved effective catalyst lives and decreased temperatures at a given conversion per pass are sought.